This invention relates to the hydroxylation of aromatic compounds by oxidation.
The conversion of aromatic compounds, particularly aromatic hydrocarbons, to their hydroxylated derivatives is of interest as a process with commercial potential since many aromatic compounds suitable as precursors are readily and cheaply available from petrochemical feed stocks. Their conversion to hydroxyaromatic compounds such as phenol by hydroxylation would be advantageous since the products are useful for many purposes, including, in the case of phenol, the synthesis of bisphenol A or 2,2-bis(4-hydroxyphenyl) propane by reaction with acetone. m-Xylene could be similarly converted to 2,6-xylenol, an essential intermediate for the production of the engineering thermoplastic poly(2,6-dimethyl-1,4-phenylene ether). Trimethylbenzenes could be converted to various hydroxy compounds useful as intermediates for such compounds as vitamin E.
A principal obstacle to commercial use of such processes has been the need to synthesize large amounts of metal-containing oxidants for use in hydroxylation. It is known that oxidative hydroxylation may be accomplished with the use of various vanadium peroxo compounds. Reference is made, for example, to Mimoun et al., J. Am. Chem. Soc., 105, 3101-3110 (1983), which discloses the use as a hydroxylation agent of such compounds as peroxovanadium oxypicolinate dihydrate, a chelate with picolinic acid (i.e., pyridine-2-carboxylic acid). This complex is prepared by the reaction of vanadium pentoxide with picolinic acid and aqueous hydrogen peroxide solution.
The hydroxylation reaction as disclosed in this and other papers apparently involves transfer of a peroxy moiety from the vanadium as an oxidant, and thus requires on the order of an equimolar amount of vanadium complex with respect to aromatic compound being oxidized. In order to be commercially feasible, a simple and effective method for regeneration of the vanadium compound would be required, thus effectively making the reaction catalytic with respect to vanadium.
A principal problem with the regeneration of the catalyst is that vanadium compounds are much more efficient at decomposing peroxide than at hydroxylating aromatic compounds. Thus, attempts at synthesis of the vanadium peroxo complex by simple addition of peroxide to the other reactants, including at least one vanadium compound, are unsuccessful since the peroxide formed is immediately decomposed by the vanadium compound rather than reacting therewith to form the peroxo complex. Various methods for overcoming this problem have been proposed, including phase transfer systems, the use of polymeric supports and slow or multiple additions of peroxide. None of these, however, has succeeded in producing a vanadium-containing promoter which is effective over a relatively long period in producing a hydroxylation product at a reasonable rate and with a reasonable degree of efficiency.
It is known that hydrogen peroxide can be produced by oxidation of hydroquinone homologs such as anthrahydroquinone. Reference is made, for example, to Heydorn et al., CEH Product Review--Hydrogen Peroxide, SRI International, pp. 741.5000H-M (August 1996). However, this method is expensive by reason of the cost of anthraquinone compounds, and any applicability of such reactions to the hydroxylation of aromatic compounds has apparently not been disclosed.